Heat assisted magnetic recording (HAMR) has been proposed as a means by which the recording density of hard disc drives may be extended to 1 Tb/in2 or higher. Current conventional hard disc drive technology is limited by the superparamagnetic limit, which causes the small magnetic grains needed for high density recording media to gradually lose their magnetization state over time due to thermal fluctuations. By using heat assisted magnetic recording, the magnetic anisotropy of the recording medium, i.e. its resistance to thermal demagnetization, can be greatly increased while still allowing the data to be recorded with standard recording fields. A laser beam heats the area on the disc that is to be recorded and temporarily reduces the anisotropy in just that area sufficiently so that the applied recording field is able to set the magnetic state of that area. After cooling back to the ambient temperature, the anisotropy returns to its high value and stabilizes the magnetic state of the recorded mark.
The main difficulty with HAMR has been discovering a technique that is able to conduct sufficient light energy into the recording medium to heat it by several hundred degrees, but only in the area that is desired to be recorded, which typically will have dimensions on the order of 25 to 50 nm if the recording density is 1 Tb/in2. If the optical hot spot is larger than this area, it will extend to neighboring bits and tracks on the disc, and by heating those areas as well, the data recorded in those areas will be erased. Confining the optical spot to an area that is much smaller than a wavelength of light, and well below the so-called “diffraction limit” that can be achieved by standard focusing lenses, is an area of study called “near field optics” or “near field microscopy.” Techniques have been described in the literature for confining light to 20 nm spots or smaller. However, these techniques have not demonstrated a capability for delivering a substantial amount of optical power to the sample within that small spot.
U.S. Published Patent Application No. 2004/0001394 A1 described a technique for delivering a large amount of optical energy into a spot of 50 nm diameter or smaller which would be suitable for HAMR. However, it would still be desirable to increase the efficiency of coupling light energy into the recording medium in order to improve manufacturing margins and enhance disc drive performance.
This invention provides a transducer design that theoretically provides improved coupling efficiency.